Carbon/carbon heat exchanger and manufacturing method

ABSTRACT

A heat exchanger includes a plurality of plates, and a plurality of sharp-cornered carbon/carbon fins between the plates.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of U.S. Ser. No. 08/601,754 filed Apr. 12, 1996, which was a continuation-in-part of U.S. Ser. No. 08/831,457 filed Mar. 31, 1997, now abandoned, which was a continuation of U.S. Ser. No. 08/422,334 filed Apr. 13, 1995, now abandoned.

BACKGROUND

[0002] The present invention relates to heat exchangers. More specifically, the present invention relates to carbon/carbon heat exchangers.

[0003]FIG. 1 shows a conventional metal parallel plate heat exchanger 10. The heat exchanger 10 includes flat plates 11 a-11 h and fins 12 a-12 b, which separate the plates 11 a-11 h. The plates 11 a-11 h and fins 12 a-12 b form hot and cold passageways 13 and 14. During operation of the heat exchanger 10, a hot fluid is circulated through the hot passageways 13, a cold fluid is circulated through the cold passageways 14, and heat is transferred from the hot fluid to the cold fluid. The fins 12 a-12 b help transfer the heat. Increasing surface area of the fins 12 a-12 b increases the heat transfer efficiency of the heat exchanger 10.

[0004] The fin surface area may be increased by increasing the number of fins per inch. Reducing the fin radius allows a greater number of fins to be formed per unit length. Increasing the number of fins per unit length, in turn, increases the heat transfer efficiency of the heat exchanger 10.

[0005] In an aircraft environmental control system (ECS), a conventional high-temperature heat exchanger is made of steel. The steel has a moderate thermal conductivity, and it can withstand high temperatures and pressures that occur during normal operation of the environmental control system. Steel fin fabrication is highly advanced: complex geometries that are highly efficient can be made.

[0006] Carbon/carbon composite is a desirable material for ECS heat exchangers. The carbon/carbon composite has a high strength-to-weight ratio (it is lightweight yet strong). Carbon/carbon composites may be made from high modulus fibers. The high modulus fibers have a high thermal conductivity, which is desirable for heat exchangers.

[0007] However, the highly efficient, complex geometries of steel fins have not yet been demonstrated in carbon/carbon composites.

[0008] There are a number of technical challenges involved in fabricating fins for carbon/carbon heat exchangers. One challenge is corrugating woven sheets of high modulus fibers into fins having tight radii. The high modulus carbon fibers are stiff and difficult to bend. Certain fibers might bend into fins having large radii, but attempting to bend the stiff fibers into tight radii fins could cause the fibers to break.

SUMMARY

[0009] A heat exchanger includes a plurality of plates; and a plurality of carbon/carbon fins between the plates. The fins are sharp-cornered, having tight radii. Reducing the radii allows for a higher number of fins per unit length. Increasing the number of fins per unit length can increase heat transfer efficiency of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an illustration of a conventional metal parallel plate heat exchanger.

[0011]FIG. 2 is an illustration of a carbon/carbon parallel plate heat exchanger according to the present invention.

[0012]FIGS. 3a and 3 b are illustrations of tools for forming thin woven carbon-based sheets into heat exchanger fins.

[0013]FIGS. 4a and 4 b are illustrations of plain weave and open weave carbon preforms that can be used for the heat exchanger according to the present invention

DETAILED DESCRIPTION.

[0014] Referring now to FIG. 2, a carbon/carbon parallel plate heat exchanger 20 includes a plurality of flat carbon/carbon parallel plates 21 a-21 h. Carbon/carbon fins 22 a-22 b are disposed between the plates 21 a-21 h to separate the plates 22 a-22 b for fluid flow and to add rigidity to the heat exchanger 20 as a whole. Hot side passageways 29 and cold side passageways 30 are formed by the plates 21 a-21 h and fins 22 a-22 b. A hot fluid 23 flows through the hot side passageways 29 and a cold fluid 24 flows through the cold side passageways 30. At least one of the fluids 23 and 24 may be air.

[0015] The cold side passageways 30 are most frequently oriented to facilitate the flow of the cold fluid 24 transverse to the flow of the hot fluid 23 in the hot side passageway 29. The passageways 29 and 30 may instead be oriented in parallel to provide the parallel flow stream arrangement of a counterflow heat exchanger. In this instance special provisions are added to assist the fluid entry and exit. In a preferred embodiment the plates 21 a-21 h can be stacked to form alternating passageways 29 and 30 until the assembly as a whole provides the required heat transfer or exchange capability.

[0016] Fabrication of a compact carbon/carbon parallel plate heat exchanger from carbon based composite materials preferably includes a specific sequence of manufacturing steps described below. Generally the process begins with a material comprised of low modulus pitch carbon fibers which can be readily formed into a shape suitable for either the plates 21 a-21 h or the fins 21 a-21 b and then, after a heat exchanger core assembly has been constructed from the component parts, the material of the assembled structure is heat treated to obtain a finished core exhibiting the desired properties. It is preferred that woven sheets be used for the plates 21 a-21 h and fins 22 a-22 b. The plates 21 a-21 h may be fabricated from any open weave sheets. A densification treatment (described below) eliminates substantially all porosity in the plates 21 a-21 h and increases thermal conductivity substantially. The fins 22 a-22 b can be a tape or cross weave of minimum thickness that can be formed into the desired corrugated fin structure. A preferred thickness of the woven sheets for the plates 21 a-21 h may be in the range of 0.010-0.020 inches and the thickness of the woven sheet for the fins 22 a-22 b may be in the range of 0.003-0.010 inches.

[0017] A preferred method of fabrication can comprise the following steps described in detail below. A preform of low modulus pitch carbon fibers, which can be readily formed to a shape desired for either the plate or the fins, is provided. A preferred preform is a woven sheet of low-cost, low-modulus pitchbased carbon fibers, for example Amoco P30X or Nippon XNC25. The woven sheet may be any of several weave configurations selected to give the desired thickness and surface characteristics. For instance a preform illustrated in FIG. 4a having a thickness in the range of approximately of 0.010-0.020 inches and a plain weave has been found to provide acceptable results for the plates. FIG. 4b illustrates a sheet having an open weave and a thickness of in the range of approximately 0.003-0.010 inches that can be used to fin plates in combination with the tools of FIGS. 3a and 3 b.

[0018] Such woven sheets are preferably impregnated with an appropriate resin, for example, phenolic or epoxy novellac by a pressure rolling process in which for instance the sheets are squeezed between opposed rollers. The resin can be applied to the sheets either before of during rolling. The resulting sheets are in a state where they can be handled and formed when they leave the rollers. The quantity of resin is carefully controlled to provide the desired state after pyrolisis. During pyrolisis the bulk of the resin is burned off and the remainder is converted to carbon. The desired quantify of resin residue is no more than approximately 20% of the original resin or preferably 3-11% of the combined volume of composite fiber and resin structure. The carbon residue remaining from pyrolisis resin does not convert to the high conductivity properties of the fibers and should therefore be the minimum necessary to hold the structure together.

[0019] Some treated sheets can be used for the plates 21 a-21 h and other treated sheets can be used to form the fins 22 a-22 b. The fins 22 a-22 b are preferably shaped as corrugated fin structure. The plates 21 a-21 h are preferably formed from the same type woven sheets and impregnated with the same resin type resin as the fins 22 a-22 b.

[0020] The resin-impregnated woven fin sheets are pliable and can be shaped and formed into the desired fin geometry. FIGS. 3a and 3 b illustrate two tools that can be used to form the desired corrugated fin sheets 22 are shown. A press fit tool shown in FIG. 3a includes a machined plate 40 having a series of articulated substantially parallel metal fingers 42 defining grooves 44 for receiving the fin weave therein. The number of grooves/fingers per inch define the fins per inch and fin height of the resulting corrugated fin structure. The woven, impregnated fin sheets 22 can be pressed into the grooves preferably using Teflon insertion strips.

[0021] In the alternative, an interleaved tool 50 illustrated in FIG. 3b may be used to wrap the resin impregnated woven fin sheets 22 around the edges of each metal plate or leave 57, 52 to form a corrugated fin sheet 22. The first leave 57 is moved right toward the center and the second leave 52 is moved left toward the center, forming a corrugation in the fin sheet 22. As successive leaves are moved toward the center additional corrugations are formed. The number of grooves/fingers per inch define the fins per inch and the fin height of the resulting corrugated fin structure is defined by the distance that the leaves 57, 52 overlap.

[0022] The resin impregnated carbon fins 22 a-22 b are then stacked between and into an array of parallel plates 21 a-21 h to achieve the structure shown in FIG. 2. However the alternating fin sheets may be of different geometries and the weave of the plate sheets and the fin sheets may also be different.

[0023] The stacked structure is then cured in a low temperature oven approximately 200-300° C. to bond all the carbon/carbon pieces together. The curing is preferably done by slowly heating the impregnated structure at a rate no more than 2° C./mm to a curing temperature in the range 180-250° C. for a time in the range 20 mm to five hours, and preferably under a compressive stress of 0.012 Mpa (mega pascals).

[0024] This impregnated and cured carbon fiber resin structure is then pyrolyzed according to a prescribed temperature-time schedule. A typical pyrolytic schedule includes heating the cured structure in an inert atmosphere or vacuum at a rate of 1-5° C./mm. (preferably 2° C./mm.) to a temperature in the range 900-1000° C. for a time in the range 30 minutes to 5 hours. The impregnation and curing/pyrolisis results in a structure having a desired shape and increased rigidity compared to the starting sheets. The structure is strengthened to facilitate additional handling as well.

[0025] The next step is densification of the structure by chemical vapor infiltration and chemical vapor deposition (CVI/CVD) of carbon, to achieve an overall bulk density in the range 1.70-2.20 g/cm³ and a microstructure of deposited carbon, for example rough-laminar carbon. Such densification may be carried out in a hot-wall, isothermal, isobaric CVI reactor, at temperatures in the range 900-1100° C., using carbon-containing vapor or gaseous precursors such as methane, ethane, ethylene, propane, butane, pentane, cyclopentane or hexane. A specific example may involve the use of flowing cyclopentane vapor (C₅H₁₀), either pure or diluted in argon, in the temperature range 1000-1100° C. to achieve densities in the range 1.70-1.95 9/cm³ in less than 12 h, with an overall conversion efficiency in the range 6-10% (conversion efficiency is defined as the amount of carbon incorporated as a solid in the preform divided by the amount of carbon introduced into the reactor in the precursor). It is intended that the CVD/CVI treatment (a) fill the residual voids of the carbon/carbon preform with carbon and thereby achieve a density which is sufficiently high to ensure the desired mechanical, thermal and permeation/leak tightness properties of the composite after additional treatment, and (b) fill the residual voids with carbon having the desired microstructure, for example, rough-laminar, which after appropriate further annealing heat treatment will become more ordered and more graphitic and hence have the desired high thermal conductivity.

[0026] After densification the structure is annealed in the temperature range 1800-3000° C. to further improve the physical properties of the structure, by making both the fiber carbon and the matrix carbon present in the structure more orderly (rendering them more graphitic). This increases the in-plane and through plane components of the thermal conductivity by orders of magnitude, obtains the desired mechanical properties, and improves the oxidative stability.

[0027] For some high temperature applications the resulting carbon/carbon structure can be further treated to infiltrate and/or coat the carbon/carbon structure with appropriate materials, which after proper heat treatment will provide sufficient oxidative stability and resistance and improved leak tightness for the intended applications. For example, a phosphates-based solution can be used to coat/infiltrate the carbon/carbon structure and after being heated treated to about 700° C. in an inert atmosphere. Such a coating may provide the carbon/carbon structure sufficient oxidation resistance for application at moderately high temperature.

[0028] SiC coatings, functionally-graded (SiC)˜C₁ . . . ˜coatings, S1 ₃N₄ coatings and other similar materials, alone or in combination, may be applied into and onto the carbon/carbon structure to further improve its oxidation resistance and stability, either alone or in combination with phosphates-type coatings.

[0029] The present invention is not limited to the specific embodiments described and illustrated above. Instead, the present invention is construed according to the claims that follow. 

1. A heat exchanger comprising a plurality of plates; and a plurality of sharp-cornered carbon/carbon fins between the plates.
 2. The heat exchanger of claim 2 , wherein the plates are carbon/carbon plates.
 3. An article for a plate-fin heat exchanger, wherein the article is a carbon/carbon fin having a sharp-cornered geometry. 